The invention relates to a method for operating an exhaust gas aftertreatment device of an internal combustion engine according to the preamble of claim 1 as well as to an open-loop and/or closed-loop control device according to the preamble of claim 11.
Ever more stringent emission standards increasingly require the use of so-called SCR catalytic converters (selective catalytic reduction) for the reduction of nitrogen oxides in exhaust gas aftertreatment apparatuses. In the case of SCR catalytic converters, a reducing agent (e.g. a urea-water solution) is introduced when needed into the exhaust gas duct of an internal combustion engine in order to make the catalytic reduction of nitrogen oxides possible. In so doing, ammonia (NH3) accrues, which reacts in the SCR catalytic converter with the environmentally harmful nitrogen oxides (NOx) of the exhaust gas and converts these into non-toxic water (H2O) and nitrogen (N2).
The SCR catalytic converter, however, only begins to convert nitrogen oxides at a certain operating temperature (approximately 180-200 E). The internal combustion engines of motor vehicles are, however, frequently (colloquially “cold”) started at a lower ambient temperature of approximately 20 EC. In order to bring the SCR catalytic converter as quickly as possible to its operating temperature, the practice of triggering active heating measures is known. In addition, heat is produced by measures taken inside the engine as well as by exothermal reactions of unburnt hydrocarbons in the oxidation catalytic converter, which bring about an accelerated heating of the entire exhaust gas duct and also thereby of the SCR catalytic converter. During the exothermal process, oxygen oxidizes hydrocarbon (e.g. C2H6) into harmless carbon dioxide (CO2) and water (H2O), and in doing so releases heat. The hydrocarbons reacting in the oxidation catalytic converter are introduced into the exhaust gas aftertreatment apparatus by a late afterinjection, which no longer combusts in the combustion chambers, or also directly into said exhaust gas aftertreatment apparatus.
These heating measures can be controlled purely in an open loop without feedback. An alternative solution is to implement the heating measures as a closed-loop control in order to achieve a certain temperature profile.
The German patent publication DE 102 58 278 A1 describes a method for modeling a catalytic temperature during an exothermal operation. The exothermal process is thereby triggered for regenerating the catalytic converter, respectively a particle filter. In the known method, a calculation of the temperature of the catalytic converter for the normal operation without exothermal influences as well as for the operation having a regeneration of the catalytic converter, which takes place exothermally, is carried out. The known method takes a temperature of the catalytic converter or the particle filter into account when controlling the internal combustion engine in connection with a regeneration of said catalytic converter or said particle filter. The objective is thereby to prevent the internal combustion engine from being operated with an excess amount of fuel, e.g. in the case of insufficient exhaust gas temperature, in order to trigger a regeneration. If on the contrary an allowed maximum value for the temperature of the exhaust gas is exceeded during a regeneration taking place exothermally, counter measures are introduced.
Methods similarly configured for operating an exhaust gas aftertreatment apparatus are described, for example, in the German patent publications DE 10 2004 031 321 A1 and DE 10 2005 054 579 A1.
Irrespective of the type of heating measures, the hydrocarbon conversion capacity of the oxidation catalytic converter is not taken into account by conventional applications. When the oxidation catalytic converter has a limited hydrocarbon conversion capacity, the hydrocarbons metered in for heating purposes cannot be completely converted by said oxidation catalytic converter, which then leads to a hydrocarbon slip. The unreacted hydrocarbons can also not be converted in the particle filter, which is usually disposed immediately downstream of the oxidation catalytic converter, due to the lower temperature of said particle filter. Said unreacted hydrocarbons consequently reach the SCR catalytic converter, which is usually disposed downstream of said oxidation catalytic converter and said particle filter. The hydrocarbons occupy storage locations there, which are needed for the storage of ammonia (NH3), respectively nitrogen oxide (NOx). The result of this effect, which is also denoted as hydrocarbon poisoning, is a reduction in the nitrogen oxide conversion and thereby an increase in the emission of nitrogen oxide.
It is also disadvantageous that the hydrocarbon poisoning can lead to misdiagnoses of the SCR catalytic converter. The term misdiagnosis is used in this case because said SCR catalytic converter is diagnosed to be defect due to the hydrocarbon poisoning although the situation relates to a reversible poisoning. During a regeneration of the particle filter, the hydrocarbon is expelled from said SCR catalytic converter. Said SCR catalytic converter is thereafter again fully functional. Permanent damage to said SCR catalytic converter is therefore not present. A diagnostic result, which judges said SCR catalytic converter to be defective, would therefore represent a misdiagnosis.
The hydrocarbons, which do not remain in the SCR catalytic converter and occupy storage locations there, leave the exhaust gas aftertreatment apparatus and also contribute to an increase in the hydrocarbon emissions in addition to the increase in the nitrogen oxide emissions.